Densification of hard chrome plated surfaces

ABSTRACT

Porous chrome plated surfaces are densified by treating the surface with a chromic acid solution and heating the part to cure and bring about the conversion of the chromic acid to chromic oxide. The procedure may be repeated as required until the pores and cracks have been filled.

llmte States Patent 1 1 1111 3,849,206

Jacobs Nov. 19, 1974 [54] DENSIFICATION OF HARD CHROME 2,438,013 3/1948Tanner... 204/35 R x PLATED SURFACES 2,618,578 11/1952 Kreml 148/62 X2,746,915 5/1956 Giesker et al 204/35 R Inventor: Louis John Ja o s,cago, 111. 2,755,242 7/1956 Keller et al. 1. 204 35 R x 2,773,62312/1956 Schuster et al. 148/612 X [73] Ass'gnee' cqmgustmn 3,526,486 91970 Smith et al 204/35 R x Wm 3,755,091 8/1973 Austin 204 35 R x [22]Filed: Jan. 2, 1973 2 Primary Examiner-Ralph S. Kendall [21] Appl-320,217 Attorney, Agent, or Firm-Richard H. Berneike [52] US. Cl.148/6.2, 204/35 R, 204/38 R ABSTRACT [51] Int. Cl. C23f 7/26 Porouschrome plated surfaces are densified by treat- [58] Field of Search148/62; 204/35 R, 38 S, ing the surface with a chromic acid solution andheat- 204/38 R ing the part to cure and bring about the conversion ofthe chromic acid to chromic oxide. The procedure [56] References Citedmay be repeated as required until the pores and UNITED STATES PATENTScracks have been filled.

2,327,885 8/1943 Grace et a1 117/71 M X 3 Claims, N0 DrawingsDENSIFICATION OF HARD CHROME PLATED SURFACES BACKGROUND OF THE INVENTIONThere are many commercial applications for hard chrome plated parts suchas refractory patterns, dies for forming plastic and ceramic parts,hydraulic actuators, cylinder liners, and ball and check valves as wellas many other uses. All of the present commercial chrome electroplatedsurfaces of metal parts contain micropores and a chicken wire crazingpattern of cracks which are visable with a low power microscope. It isthese inherent defects which cause destruction of the chrome platedsurface and failure of the part. Although chromium is very resistant tothe corrosive action of many chemicals, such as corrosive acids andalkalies, these chemicals penetrate the pores and cracks and corrode themetal substrate. This action results in the flaking or pealing of thechromium plating from the substrate and parts which are subject to veryhigh pressures, such as hydraulic components, may leak after a shortservice period.

SUMMARY OF THE INVENTION The object of the present invention is to treatchrome plated surfaces to densify the surface by filling in inherentdefects such as micropores and cracks. This densitication will preventpenetration of the chrome plating by chemicals and thereby reduce thetendency of the chrome plating to become detached from the substrate andreduce chemical attack on the substrate. The objects of the presentinvention are accomplished by treating the chrome plated surfaces with achromic acid solution and then converting the chromic acid to chromicoxide.

DESCRIPTION OF THE PREFERRED EMBODIMENT The chromic acid solution foruse in the present invention isprepared by dissolving chromium trioxidein water to produce a chromic acid solution having a specific gravity of1.64-1.66 at 70F. The preferred specific gravity is 1.65. The chromeplated metal part to be treated is then immersed into the chromic acidsolution or otherwise treated with the solution and the chromic acidwets and penetrates the micropores and cracks of the chrome platedsurface. The treated metal part is then heated so as to convert thechromic acid to a refractory chromic oxide. The conversion of thechromic acid to chromic oxide takes place at a temperature between 700Fand 1,200F. However, a preferred heating cycle should be followed toobtain the most effective densification. This heating cycle involvesfirst heating the treated metal part to 230F t F and held at thistemperature for a period of time sufficient to insure that all themoisture is removed from the micropores and cracks of the chrome platedsurface. The part is then gradually brought to a temperature of 700F i50F. The rate of heating to this temperature level is determined by themass and complexity of the metal part and is selected to avoid thermalstresses. Sufficient time is allowed to stabilize the temperaturethroughout the metal substrate. After the temperature has stabilized at700F i 50F, the temperature of the surface is rapidly brought to 1200F 50F and then the part is removed from the heating equipment. This rapidheating from 700 to 1,200F converts the chromic acid to chromic oxideand the chromic oxide becomes very dense and is more strongly bonded tothe base metal.

The repeated coating with chromic acid and curing to form chromic oxidewill tend to more completely fill the pores and cracks of the chromeelectroplated surface. After the first coating has been applied andcured, the part is then cooled to F and again treated with chromic acidsolution, heated to 230F, gradually brought to the stabilizedtemperature of 700F and then rapidly heated to 1,200F. This procedure isrepeated until the pores and cracks have been filled. The part may thenbe finished by polishing by conventional means.

Heating of the metal parts may be accomplished by any conventional meanssuch as in ovens or furnaces, or by means of radiant heat burners or bydirect flame. However, parts which have been heated with inductionheating have proved to be the most effective in bonding the chromicoxide to the substrate. This fact is apparent on a microscopicexamination of sections of parts treated in accordance with the presentinvention.

It has been found to be helpful in the practice of the present inventionto add about 0.05 to 0.5 percent by weight of carbon black or lamp blackbased on the weight of the chromic acid used to make the chromic acidsolution. The presence of the carbon reduces the oxygen content of thesolution and in some manner in creases the rate of filling of the poresand cracks with chromic oxide.

Although the present invention has been described with reference tospecific examples it will be understood that these are merelyillustrative and that changes may be made without departing from theinvention as claimed.

What is claimed is:

l. A method of treating metal parts having chrome plating thereonwherein said chrome plating contains pores and cracks comprising thesteps of:

a. applying a solution to said chrome plated metal parts wherebysolution penetrates said pores and cracks, said solution consistingessentially of a chromic acid solution having'a specific gravity of from1.64 to 1.66; and

b. heating said chrome plated metal parts having said chromic acidapplied thereto to a temperature of about 230F- 10F, graduallyheating toa temperature of 700F 50F, permitting said temperature to stabilize at700F 50F throughout said part and then rapidly heating said part to1,200F to convert said chromic acid to chromic oxide.

2. A method as recited in claim 1 wherein said step of heating comprisesthe step of heating by induction heating.

3. A method as recited in claim 1 wherein said chromic acid solutioncontains a material selected from the group consisting of carbon blackand lamp black, said material being present in an amount equivalent to0.05 to 0.5 percent by weight of the chromic acid present in the chromicacid solution.

1. A METHOD OF TREATING METAL PARTS HAVING CHROME PLATING THEREONWHEREIN SAID CHROME PLATING CONTAINS PORES AND CRACKS COMPRISING THESTEPS OF: A APPLYING A SOLUTION TO SAID CHROME PLATED METAL PARTSWHEREBY SOLUTION PENETRATES SAID PORES AND CRACKS, SAID SOLUTIONCONSISTING ESSENTIALLY OF A CHROMIC ACID SOLUTION HAVING A SPECIFICGRAVITY OF FROM 1.64 TO 1.66; AND B. HEATING SAID CHROME PLATED METALPARTS HAVING SAID CHROMIC ACID APPLIED THERETO TO A TEMPERATURE OF ABOUT230*F +- 10*F, GRADUALLY HEATING TO A TEMPERATURE OF 700*F+50*F,PERMITTING SAID TEMPERATURE TO STABILIZED AT 700*+50*F, THROUGHOUT SAIDPART AND THEN RAPIDLY HEATING SAID PART TO 1,200*F TO CONVERT SAIDCHROMIC ACID TO CHROMIC OXIDE.
 2. A method as recited in claim 1 whereinsaid step of heating comprises the step of heating by induction heating.3. A method as recited in claim 1 wherein said chromic acid solutioncontains a material selected from the group consisting of carbon blackand lamp black, said material being present in an amount equivalent to0.05 to 0.5 percent by weight of the chromic acid present in the chromicacid solution.